The internal dimensions of the headbox of a paper machine determine the maximum and minimum values for the flow volume running through the headbox, values between which an acceptable web quality is obtained.
Should the flow volume exceed the maximum value, the flow speeds in the headbox become so high that the distribution of stock in the direction of the breadth of the headbox becomes uneven and, in addition, the output flow from the headbox as it touches the wire is too turbulent to form a satisfactory web.
If the flow volume is below the minimum value, the stock may be unevenly distributed in the direction of breadth of the headbox. However, the turbulence below the minimum value is too weak, and the fibres start fastening on one another forming fibre bundles, i.e. flocs.
These extreme limits are diffuse to some extent yet they are so clear that their existence is generally known.
Paper's weight grade or mass divided by area (the so called grammage) is dependent on the machine speed and the consistency of the stock flowing from the headbox. Machines that produce grades with very different grammages, like boxboard machines, have large speed and consistency ranges. This is due to the fact that attempts are made to produce light grades at a low consistency in order to achieve good formation and at a high speed to maximize production. In order to produce heavy grades, the consistency must be increased and the machine speed reduced because the other parts of the paper machine usually set the limits for increased production. Since the slice flow volume is directly proportional to speed and in inverse relation to the consistency, the relation of the maximum and minimum flow volume of a headbox of this kind of machine is high. In the following, this relation will be referred to as flow ratio (S), and it can be presented in a formula: EQU S=Qmax/Qmin
in which
S=flow ratio PA1 Qmax=the highest flow-through volume applicable to a headbox which gives an acceptable web quality and sufficient runnability PA1 Qmin=the lowest flow-through volume on corresponding conditions PA1 equipped with a tubular type turbulence generator PA1 equipped with a partition plate type turbulence generator PA1 as above PA1 both types of turbulence generator can be installed in the same headbox so that, for example, the middle layer uses high-consistency stock and a turbulence generator with a partition plate, and the outer layers use conventional consistencies and a tubular type of turbulence generator.
The flow ratios (S) attainable with the present headbox structures have, in several cases, turned out to be insufficient to guarantee an acceptable quality of the final product at the extreme limits of the grammage range. The flow ratio of a headbox should be at least as high as the relation of the highest and lowest running capacity of the paper machine. The best result, however, is reached if the headbox flow ratio is increased to such an extent that the range of the real flow volume running through a headbox is clearly within the flow range determined by the flow ratio.
In rectifier roll headboxes, the flow ratio (S) is approximately 2.5. The weak point of a hydraulic headbox is a smaller control range, its flow ratio (S) varies between 1.5 and 2.0 depending on the conditions.
It is previously known that to the both above mentioned types of headbox additional features have been applied with a purpose to adjust the flow ratio of the headbox in question, or, in some cases, only to increase/decrease the amount of stock or water in the slice chamber in order to correct the local defects occurring in the slice flow to produce a better web quality.
The prior art solutions, however, have shortcomings that the present invention will eliminate. A typical defect is that the realized flow control method changes the flow speeds in the entire headbox. Another weakness is that the closing of some channels of a headbox causes danger of clogging and thus the access of fibre bundles onto the wire. The third defect is the arrangement of the by-pass in a manner that the pattern of turbulence in the slice chamber undergoes a fundamental change. The fourth defect comparable with the latter one is that a decreased flow volume is directed to a slice chamber with constant dimensions where the reduced flow speed spoils the turbulence. A fifth defect is the impractical mechanical solutions for achieving the required adjustments.
The present invention solves all of the above mentioned five shortcomings and, in addition, gives an opportunity to use the same headbox for the production of a multiply web.